Some evidence supports the hypothesis that the mutation rate at a particular site is influenced by neighboring base-pairs. I have sought to modify the reversion and conversion frequencies of bacteriophage T4 rII nonsense codons by inserting nearby temperature-sensitive rII lesions. The insertion of a ts mutation reduces the 2-aminopurine-induced reversion of an rII amber mutation about three-fold. The ts mutation reduces the corresponding UAA>UGA conversion about eight-fold, while the reversion of the ochre codon to glutamine (UAA>CAA) is not affected. Selection controls show no measurable selection against the ts marker.